1. Field of the Disclosure
This disclosure relates to low temperature cooling devices. Particularly, this disclosure relates to a gas exchange housing for low stress and strain mounting of a sample while reducing vibrations transferred to, and providing optical access to, the sample.
2. Description of the Related Art
Cryogen-free refrigerators, also called cryo-coolers or cryostats, utilize a closed-cycle circulating refrigerant (often helium gas) to extract heat from a cold finger at cryogenic temperatures and pump it away to a heat exchanger. The cold finger is a metallic heat sink which is actively cooled by the refrigerant. The cold finger can be temperature controlled and serves as a mounting point for an object. The main goal of attaching the object to the cold finger is that the object will be cooled. Objects to be cooled can include semiconductor devices, detectors, mechanisms, material samples, or any other objects that require fixed, cryogenic temperature operation.
While direct contact coupling of the object can cool the object (i.e., cooling is where heat is transferred from the object to the cold finger), in some applications, direct contact alone does not provide adequate performance, because the mechanical operation of the cryo-cooler couples energy in the form of vibrations, acoustic noise, or other into the object via the thermal contacts of the object. In these applications, it is necessary to delicately mount the object such that it is cooled but isolated, or not disturbed, by cryo-cooling the apparatus.
FIGS. 18A-D are diagrams of the mounting of a sample in a open cycle cryostat. For explanation of the elements, see FIG. 17. Open cycle cryo-coolers are those where the coolant is liquid helium. Bonds such as indium, clamping, or grease are used for bonding a sample to the cold finger when in a vacuum. When in a gas or liquid atmosphere, the sample is held in place simply with a piece of tape or small spring.
In testing sensitive objects as mounted in open cycle cryostats, during sensitive experiments, problems are typically not observed. As is shown in FIGS. 18A-D, no negative effects on the mounted and cooled object arise due to coupling of energy from the mounting apparatus (such as that from the cold finger) during the experiment, since the mounted object is not experiencing any significant external disturbances from the transfer of liquid helium to the cold finger. Good performance in experiments is generally observed, independent of the function of the experiment, when the matter involved with the cooling has good thermal transfer, is stationary and not under any stress.
When moving from an open cycle system to a closed cycle system, one major change is seen. In an open cycle system, there is no force on the representative cold finger. However, in a closed cycle system a mechanical noise/force is present. If one uses mounts created for an open cycle cryostat the forces are coupled directly into the sample as shown in FIGS. 19A-D. Closed-cycle cryo-coolers or cryo-refrigerators are mechanical devices which provide cooling to a cold finger via pressure cycling of gas.
In a conventional closed-cycle cryo-cooler, the sample is mounted to a cold finger in a vacuum, where good thermal contact is required. The thermal contact is ideally provided by a physical mount with substantial contact area and large thermal conduction with the sample. Good thermal contact is usually achieved with firm pressure and intermediary grease or other filling-material. Indium can be used to attach the object and the metal cold finger. However, a stiff contact from firm pressure can too easily couple vibrations or induce undesired stresses in sensitive materials.
The main problem in a closed-cycle cryo-cooler is that mechanical work is required to produce cooling power rather than obtaining cooling from a liquid source. This mechanical power produces the expansion of compressed gas at the cold finger, and thus typically transfers vibrations to the sample mount. This repeated rhythmic hammering, although at low frequency usually 1-2 Hz, drives higher frequency resonances inside the cryostat. High frequency, high intensity vibrations can also be created from the gas flow through the head across sharp corners or other imperfections in the piping, thereby creating noise that will interfere with a sensitive experiment. High intensity, high frequency vibrations couple into a sample through the physical contact of a mount. These high frequency vibrations produce a very unstable mounting platform for sample mounts. The best conventional sample holders in closed cycle cryostats coupled the least amount of vibrations into a crystalline sample, but still had inadequate performance.